These frogs are not aggressive.

However, much like fish, aquatic frogs have sensory organs which allow them to detect movement in the water. Hymenochirus have poor vision so they rely on this greatly to hunt. It is more than likely your frogs detected the movement of the other and mistook it for prey, mistaken identity happens a lot with aquatic frogs, it's harmless behavior. I've seen my frogs bite each others legs and arms, it happens but it's not something to worry about, they realize it's another frog and let go (of course cannibalism does happen with amphibians often but this is not the case with your situation).

If they both latched on to a blood worm at the same time, they will struggle and try to take it from the other, give two lions a piece of steak and the same thing happens, it's animal instinct. I saw my two Xenopus Laevis struggle over an earthworm as if their lives depended on it and wound up ripping it in two, they are not aggressive towards another but if they both have a hold on prey item, they will 'fight' the other for it, such is nature. Think of how dogs will play tug of war with a rope toy, more or less the same scenario.

@Caspian: Hymenochirus are not carriers of Chytrid to my knowledge, they are as susceptible as the next frog to the fungus. A similar frog, Xenopus (African Clawed Frog), a larger more aggressive version of the frogs above can carry it, though they are immune.

Depending on the type of bullfrogs you keep they may also be carriers but also immune to the fungus because American Bullfrogs are also known to carry Chytrid but themselves be immune to it's effects.

Oh, and aquatic frogs aren't boring. African Dwarf frogs are kind of slow witted but the larger Xenopus are great pets and very entertaining, always active, always hungry, very comical and fun to observe!